Interferometric measuring systems with optical probes as can be arranged for the practice of our invention provide for measuring localized surface features, geometric surface forms, and overall dimensions. The invention is particularly applicable to the measurement of cylindrical, conical, and flat surfaces whose roughness approaches tolerances for geometric form as well as to the measurement of test pieces having multiple surfaces requiring individual or comparative measurements.
Tolerances for many precision manufactured components continue to go beyond the capabilities of conventional contact measuring techniques. Optical measuring techniques, particularly those using interferometric mechanisms, provide for measuring with much greater precision. However, the roughness of the surfaces under test often exceeds one-half of the wavelengths used in conventional interferometers (i.e., wavelengths in the visible or near-infrared range). Surface features larger than one-half the measuring wavelength cannot be unambiguously measured with conventional interferometers. Longer wavelengths can be used, but lasers for producing such longer wavelengths are less common and more expensive than those available for producing wavelengths in the visible or near-infrared range.
Manufactured components that include multiple surfaces can require measurements of their individual surface forms (e.g., roundness and straightness) as well as measurements of relationships between their surfaces (e.g., runout and perpendicularity). Measuring each of the surfaces individually with setups or recalibrations between the different measurements is time consuming and can make comparisons difficult.
Our interferometer in one or more of its preferred embodiments provides for measuring multiple surfaces with a compound optical probe. Sub-test beams emitted from the probe separately measure the multiple surfaces. A confocal optical system distinguishes the measurements between the surfaces. Each of the sub-test beams can be composed of two fundamental wavelengths of light from different interferometers. Combined, the two interferometers greatly increase the dynamic range of measurement for measuring rough surfaces with conventional lasers.
An exemplary interferometer for measuring multiple surfaces of a test piece in accordance with our invention includes a test arm and a reference arm that convey test and reference beams along different but ultimately interconnected paths. A beamsplitter within the test arm separates the test beam into first and second sub-test beams. A focusing optic of the confocal optical system within the test arm focuses the first and second sub-test beams to different points of focus. A compound probe also within the test arm conveys the first and second sub-test beams to the different points of focus.
Preferably, each of the sub-test beams is intended for measuring a different surface of the test piece at normal incidence. As such, the principal axes of the sub-test beams are oriented normal to their incident test surfaces, which can be oriented in different directions. Directional optics within the probe direct the sub-test beams to their points of focus at their intended orientation. Additional sub-test beams can be split from the test beam within the test arm for measuring more than two surfaces of the test piece, each being directed to a point of focus at normal incidence to a different test surface.
The test surfaces are preferably measured individually in succession. An actuator relatively moves the probe with respect to the test piece between two or more measuring positions. In a preferred embodiment, the actuator is movable between two positions for measuring two surfaces of a test piece. At a first of the positions, the point of focus of the first sub-test beam is positioned on the first surface of the test piece and the point of focus of the second sub-test beam is positioned off both the first and second surfaces of the test piece. At a second of the positions, the point of focus of the second sub-test beam is positioned on the second surface of the test piece and the point of focus of the first sub-test beam is positioned off both the first and second surfaces of the test piece. Similarly, at a third or higher measuring position, the additional points of focus are positioned in turn on other of the test piece surfaces while the remaining points of focus are positioned off of all the test surfaces.
A detection system detects an interference signal between the reference beam and the first sub-test beam when the probe is located at the first position and detects an interference signal between the reference beam and the second or higher sub-test beam when the probe is located at the second or higher position. The detection system is preferably arranged in conjunction with a confocal optical system that excludes from detection light that is not focused on one of the test surfaces. An imaging optic of the confocal optical system can be used to refocus the sub-test beams conjugate to their points of focus of the focusing optic. A limited aperture size near the focus of the imaging optic limits a depth of focus through which light is effectively collected by a detector at the end of the confocal optical system. If any of the test surfaces are located out of focus (e.g., by as few as 10 to 100 microns), little of the reflected light reaches the detector. The aperture size can be limited by locating a stop near the conjugate focal point or by locating a detector of limited dimension near the same point of focus.
For measuring rough surfaces or surfaces with significant discontinuities, such as surfaces with an average roughness approaching one-half of wavelengths in the near-infrared range, our invention provides laser sources that produce two beams having different fundamental wavelengths of light. Beamsplitters divide each of the different wavelength beams into test and reference beams. Another beamsplitter combines the two different wavelength test beams into a common test beam composed of the two different wavelengths. It is the common test beam that is divided into the multiple sub-test beams, resulting in each of the sub-test beams being composed of the two wavelengths.
Each of the different wavelength reference beams preferably propagates along respective reference delay lines of the reference arm for controlling the optical path lengths traversed by the two reference beams. Preferably, the two reference delay lines have adjustable optical path lengths to equate optical path lengths between the test and reference arms of the interferometer. The optical path lengths of the test and reference arms can also be equated by incorporating similar path-length adjustments within the test arms.
The detection system preferably includes first and second arrays of detectors for separately detecting interference between each of the two pairs of test and reference beams. The detectors within each of the first and second arrays are preferably relatively phase shifted for simultaneously detecting a plurality of phase-shifted measurements within each of the first and second pairs of test and reference beams. The simultaneous phase-shifted measurements allow for discerning more accurate phase differences between the test and reference beams at each fundamental wavelength.
Although accurate, the two individual wavelength measurements produce ambiguous results for surface discontinuities greater than one-half the fundamental wavelengths. Our invention, however, provides a controller that combines information from the first and second arrays of detectors to produce aggregate interference measurements having a sensitivity equated to an effective wavelength significantly longer than either of the two different fundamental wavelengths. The aggregate measurements are useful for measuring surfaces with a roughness exceeding one-half the two fundamental wavelengths.
The actuator is preferably a part of a relative motion system between the probe and the test piece for measuring a plurality of points on each of the two surfaces of the test piece. Preferably, both the test arm and the reference arm are relatively movable together with the probe with respect to the test piece. The detection system is also preferably mounted together with the test and reference arms and the probe on a multi-axis stage assembly for relative motion with respect to the test piece. A base preferably supports both the test piece and the multi-axis stage assembly for relating motions between the probe and the test piece. A displacement-measuring interferometer preferably measures movements between the multi-axis stage assembly and the base. Information from the displacement-measuring interferometer can be combined with interferometric measurements taken through the probe to compensate for any motion errors of the relative motion system or to resolve remaining phase ambiguities required to obtain absolute measurements.
Our preferred method of measuring multiple surfaces of a test piece with a scanning interferometer follows the basic interferometric practice of dividing a beam of light into test and reference beams but further divides the test beam into multiple sub-test beams. The multiple sub-test beams are focused to different points for separately measuring different surfaces of the test piece. For measuring a first test piece surface, the point of focus of a first sub-test beam is positioned on the first surface of the test piece while the point of focus of a second or higher sub-test beam is positioned off of their respective measuring surfaces of the test piece. For measuring a second or higher test piece surface, the point of focus of the second or higher sub-test beam is positioned on the second or higher surface of the test piece while the point of focus of the first or other lower sub-test beams is positioned off of their respective measuring surfaces of the test piece. Relative motion between the probe and the test piece is used both (a) to move the points of focus across the test surfaces for measuring a plurality of points on each of the test surfaces and (b) to move the points of focus between the sequential measuring positions.
At their respective measuring positions, the sub-test beams are retroreflected from their points of focus on the surfaces of the test piece. The retroreflected sub-test beams are preferably refocused together with the reference beam proximate to a detector. Interference signals between each of the sub-test beams and the reference beam are detected separately according to which of the sub-test beams is positioned in focus on one of surfaces of the test piece.
The refocused light of the sub-test beams is refocused conjugate to their points of focus. A limiting aperture near the conjugate plane excludes light from the sub-test beam that is not focused on one of the surfaces of the test piece. A detector for detecting the refocused light is preferably positioned behind the limiting aperture and arranged to collect only the light that passes through the limiting aperture. Alternatively, a detector with a small active area can be located near the conjugate focal plane to function as a similarly limiting aperture excluding light that focuses before or after the focal plane. The retroreflected test beams could also be refocused through a limiting aperture prior to their recombination with the reference beam remote from the detector.
While confocal optical techniques can be used to distinguish one surface from another, two-wavelength interferometry is preferably used for extending the range of dynamic measurement to accommodate rough surfaces or surfaces with significant discontinuities. Two beams of coherent light having different fundamental wavelengths are each divided into test and reference beams. The different wavelength test beams are combined in advance of the step of dividing the test beam into multiple sub-test beams so that each of the multiple sub-test beams includes the two different fundamental wavelengths.
Along the path of retroreflection, the two fundamental wavelengths are re-separated for simultaneously measuring optical path differences between the test and reference beam portions of each of the fundamental wavelengths. The optical path differences expressed by the mechanism of interference provide overlapping measurements of individual points on one or the other of the test surfaces that is in focus. Relative motion (i.e., scanning) of the point of focus across the test surface allows for the accumulation of information describing the surface. Interference information detected from both fundamental wavelengths can be combined to reveal unambiguous measurements over a much wider range extending to one-half of an effective wavelength that is significantly longer than either of the two fundamental wavelengths.
In addition, the remaining ambiguities of the combined interferometric measurements in two wavelengths can be resolved by measuring from a known point of reference the movements required for positioning the points of focus of the sub-test beams on the surfaces of the test piece. For example, the displacement-measuring interferometer can be calibrated to a master test piece and used to track the further motions required to move the probe into the measuring positions. With the positions of the probe known and the positions of the test surfaces known with respect to the probe, absolute measures of the test surfaces can be made.